I had a similar cut-out problem and my solution was to use the bitmap tracer. I could then select the border colour, in your case the green, which would be a filled green object. I pasted that over my bitmap adjusted the size and the used Arrange/combine/slice to cut out my original.
This should also give useful 'mask' shapes for almost any flat(ish) colour area with the bitmap.
This works quite well with a hi-res bitmap and complex outlines, but can result in inaccuracies
on the edges of a low- res bitmap.
Oh the settings I used for the bitmap tracer were; Photo, 50 passes, remove noise 0%, minimum area 0, both colour tolerences at 75%, 95% accuracy and 5% smoothing.
By the way, it would be nice to have a thread or perhaps a section in the Xara zone that users could give lots of settings and example results for the tracer, this would certainly help rid us of a great deal of trial and error with this utility.